Depends on how you want to define immortality. Is it a lack of aging, or incapability of dying?
In Zu Zu's case I'd say it's a moderate mix of both. She's almost certainly not completely invulnerable, but she's managed to stop aging and she seems immune to your average weapons/magic. Which is good enough, I guess.Patrolling Lore FC almost makes you wish for a Great Revision.